The use of tubes with internal fins in conventional heat exchangers is well known and design techniques for heat exchanger tubes with internal fins are well documented in the prior art. However, internal fins have not been used in radiant tubes used in furnaces. Further, because the heat transfer mechanics of heat exchanger tubes and radiant tubes are different, the known design techniques used for heat exchanger tubes with internal fins has little applicability to radiant tubes with internal fins. Accordingly, there is a need for radiant tubes with internal fins that are properly designed for more efficient heat transfer.
By way of background, a heat exchanger tube typically carries cool gas or fluid to be heated. Hot gas or fluid flows over the outside of the tube and heat is first transferred from the hot gas or fluid to the tube by convection before heat is transferred through the tube wall by conduction. Finally, heat is transferred to the cooler gas or fluid on the inside of the tube by convection. Radiant heat transfer contributes very little to this process. As noted above, fins have long been used on the inside surfaces of the heat exchanger tubes to enhance the convective heat transfer from the tube to the inside gas or fluid.
However, while the optimum design of internal fins for use in heat exchanger tubes has been investigated and documented, the design of fins for use in radiant tubes has not been explored. In short, there is no data available for the optimum design of fins used in radiant tubes and, further, because radiation plays an important function in the transfer of heat from gases inside of the tube to the tube surface, the fin designs currently available for heat exchanger tubes are relatively inapplicable to fins for radiant tubes.
Any attempt to apply heat exchanger tube fin technology to radiant tube fin technology will be unsatisfactory because the two processes work differently. Specifically, as noted above heat exchanger tubes transfer heat almost exclusively by convection. In contrast, heat from burning gas inside a radiant tube is transferred to the inside tube surface by both convection and radiation. Typically, 10%-30% of the heat from the combustion gases is transferred to the tube wall by radiation, the remaining heat being transferred primarily by convection. Heat is then transferred through the radiant tube by conduction before being transmitted to the cool outside medium primarily by radiation. Thus, the design of internal fins for radiant tubes must take radiant heat transfer as well as convection heat transfer into consideration. Internal fin design for heat exchanger tubes must take only convective heat transfer into consideration.
Further, the cool medium transported through heat exchanger tubes must be pumped. The energy required to pump the cool medium through the heat exchanger tubes is proportional to the pressure drop created across the length of the heat exchanger tube. Thus, the design of fins for heat exchanger tubes must also take into consideration the pressure drop created by the fins. In contrast, the fuel transported through radiant tubes is propelled by combustion of the fuel or gas. Thus, the pressure drop and energy required to pump the fuel through the radiant tubes is not an important factor in the design of internal fins for radiant tubes.
Accordingly, there is a need for a radiant tube fin design that enhances both convective and radiant heat transfer inside the tube. Preferably, the fin design would provide turbulent flow within the tube for enhancing mixing of the combustion gases within the tube thereby eliminating any cold layer of gas along the inside surface of the tube. Further, increased turbulence within the tube will enhance convective heat transfer from the gases to the inside surface of the tube. Further, the radiant tube fin design must also enhance radiant heat transfer from the combustion gases to the tube. Therefore, the geometries of the fins should be such that enhancement of convective heat transfer is balanced with the enhancement of radiant heat transfer.